Wrap spring clutches have been used for several decades and are commonly used to vary the torque in uni-directional devices. Two-way wrap spring clutches or bi-directional couplings have also been used where the overrunning torque in one direction is different than the overrunning torque in the reverse direction. Such devices are well known and a typical configuration is shown and described in U.S. Pat. No. 2,868,001 issued Jan. 13, 1959 to Russell. However, most prior solutions are for use in mechanisms requiring multiple revolutions of the clutch or drive mechanisms. Thus, they tend to be overly complex and expensive for use with mechanisms where the rotation is restricted to less than one revolution such as a gear system for a cutter blade in a continuous-roll printer.
In a continuous-roll printer or facsimile machine, a stepper motor may performs two functions. When the motor turns in a forward direction, a geared system unwinds paper from the paper roll and advances it so that printing can occur. When the motor turns in the reverse direction, another gear drive system engages a cutter blade to cut the printed paper from the roll. Using the same motor both for feeding paper through the printer and cutting the printed paper is economical. In the printer example described above, a simpler clutch is typically used, such as a wrap spring slip clutch with an overrunning torque, hereinafter referred to as a slip clutch. A slip clutch typically connects the gear drive system and the cutter blade. Slip clutches are used to transmit power in one direction of rotation only (called the "locking rotation") and include teeth, ratchet or spring mechanisms that lock a driven part to a driving part when the driven part is turned in the locking direction. When the rotation of the driving part is reversed, the mechanism releases, causing the driven part to stop turning while the driving part continues to turn or "overrun" the driven part.
Some slip clutches are designed with an "overrunning torque" or a mechanism that will not automatically release during reverse rotation. A slip clutch with an overrunning torque will transmit torque from the driven part to the driving part even in the reverse direction, but will slip if the torque required to drive the driven part exceeds the overrunning torque. The previously mentioned Patent, issued to Russell, discloses an invention creating overrunning torque in both rotational directions.
As an example, consider a slip clutch with an overrunning torque of 1 inch-ounce. This slip clutch will lock if driven in its locking rotation, transmitting rotation of the driving part to the driven part without slippage. In the reverse rotation, the clutch will slip if the load on the driven part exceeds 1 inch-ounce.
Causing the clutch to slip, however, requires an amount of torque equal to the overrunning torque as a friction loss. In other words, a drive motor generating 10 inch-ounces of torque in the reverse direction through a clutch that is slipping wastes 1 inch-ounce of torque required to cause the clutch to slip. The effective torque of the motor is thereby reduced to 9 inch-ounces.
The slip clutch is configured so that a reverse rotation of the stepper motor causes a locking, or forward rotation of the slip clutch. When the stepper motor and gear drive are driven in reverse, the slip clutch locks, engaging the cutter blade to slice off a piece of paper. Afterwards, the stepper motor resumes its forward rotation, causing the slip clutch to turn in reverse. The clutch, however, will not release until the torque required to continue turning the driven part exceeds the overrunning torque. Therefore, the cutter blade may be lifted, as slip clutches may be designed to have an overrunning torque greater than the torque required to lift the cutter blade out of the paper path. The cutter blade continues to lift until it reaches a stop or limit mechanism, preventing further rotation, greatly increasing the torque required to lift the blade, and causing the slip clutch to release.
Even after the blade is lifted and the clutch released the stepper motor must continue to expend energy overcoming the overrunning torque so the blade will not fall back into the paper path. The overrunning torque of the slip clutch is high compared to normal wrap spring clutches because the overrunning torque must be high enough to reliably open the cutter blades. Furthermore, the torque required to open the cutter blade is limited to the overrunning torque. This results in friction loss, is a waste of energy, and increases the cost of the printer because a larger stepper motor must be specified than is required to drive paper through the paper path for printing. Additionally, it is rare that a slip clutch has a constant overrunning torque during its lifetime because over time environmental conditions, wear, and age modify the behavior of the clutch. If the overrunning torque becomes too high, paper will not feed properly because too much of the stepper motor's torque is wasted overcoming the friction generated by the overrunning torque. If the overrunning torque becomes too low, the cutter blade will not open or may slip back down into the paper path during printing.
What is needed, therefore, is a device without any significant overrunning torque which will economically allow a gear system to transmit the full torque of a stepper motor in one direction, then at a predetermined stop position to disengage and allow the full torque of the stepper motor to be transmitted in the reverse rotation of the stepper motor.